Sanitary napkins are multi-layered structures that normally comprise a body contacting liquid-pervious cover layer, an absorbent system and a barrier layer that prevents fluid entrapped in the absorbent system to egress from the garment facing surface of the sanitary napkin. Usually, the absorbent system is a dual-layer arrangement. It includes an absorbent core whose primary function is to entrap the liquid discharge permanently and a transfer layer whose function is to collect the body exudate quickly and then meter the liquid to the absorbent core. Liquid discharged on such composite absorbent systems will rapidly ingress the transfer layer due to its highly porous network. From the transfer layer, liquid migrates toward the absorbent core by capillary pressure because of the substantial difference in wicking power between the different materials. The liquid migration is well-controlled, occurring at the rate of acceptance of the absorbent core.
The transfer layer is in the form of a sheet and it is typically made from cellulosic fibers by an air-laid process. To consolidate the porous sheet so that it can withstand the mechanical stresses normally encountered in use, without suffering a loss of structural integrity, binder is applied on both main surfaces of the transfer layer, namely on its liquid-acquisition surface that faces the cover layer and on its liquid-release surface oriented toward the absorbent core. The amount of binder applied to both surfaces is the same.
Conventional wisdom dictates that from the point of view of liquid absorption performance, the presence of binder on the transfer layer is undesirable. The binder sites that link the fibers are thought to behave as local barriers preventing liquid from freely migrating in and within the porous structure. The requirement of fast liquid penetration is particularly important for the liquid-acquisition surface of the transfer layer that should be able to take-up rapidly large amounts of liquids. Otherwise, the sanitary napkin may fail due to overflow leakage at the transfer layer level.